Generally speaking, the transmission chain of a wireless apparatus, like a mobile phone, comprises a digital stage and an analog front-end-module coupled to an antenna. The digital stage and the front-end module are mutually coupled through a digital to analog conversion stage.
In order to reduce the number of bits in the digital to analog conversion stage, a high frequency sigma-delta modulator is used.
Due to the very high clock rate of the clock signal with which the sigma-delta modulator operates, the quantization noise is pushed far away from the emission band. However, in a mobile phone, this kind of noise is polluting the cellular receive bands like the DCS, PCS, or W-CDMA bands. Thus, a great amount of filtering (70-80 dB) may be helpful in the transmitting chain.
A first conventional approach includes using a high-order single loop multi-bit digital sigma-delta modulator. The high-order structure allows the placement of the notches in the noise transfer function of the modulator. The notch frequencies are defined by coefficients in the feedback path of the modulator. A drawback of this approach is that the coefficients are implemented by multipliers that may perform the multiplication in a fraction of the sampling clock period. This leads to a limitation of the clock frequency and, in practice, the highest achievable clock frequency is in the range of 1 GHz when using conventional silicon technologies, like 65 nanometers CMOS. This limitation means that, to achieve an acceptable noise shaping, a quite high order modulator is needed. For example, a W-CDMA transmitter architecture needs two 9th order 4-bit modulators to suppress the quantization noise in the W-CDMA Rx band, with an estimated power consumption of 150 mW, which may be too high for mobile applications.
Another conventional approach proposes a FIR filter that combines the output of a direct conversion transmitter with its delayed signals to achieve a bandpass filter. However, such an approach also suffers drawbacks.
For example, to achieve a bandpass filter with a relatively narrow band and good attenuation, a great number of FIR elements must be used, with coefficients that vary in a wide range, which may be hard to realize.
As a part of the coefficients may be negative to achieve good bandpass filter characteristics, a part of the radiofrequency power produced by the digital to analog converter may be cancelled, thus the efficiency is not optimal.
As the FIR filter is used after the RF modulators, the whole delay chain of the FIR filter may operate at least twice the carrier frequency.